Clumped isotope constraints on formation environment of Triassic carbonates in Makhtesh Ramon

Despite prolonged research, the formation environments of dolomite remain debated. Previous studies have associated the apparent decrease in dolomite abundance during the Cenozoic with a global transition in marine carbonate depositional environments leading from warm, saline, shallow platforms in which dolomite formation was possibly mediated by microbial activity, to deeper and cooler environments in which dolomite formation was largely inhibited. Others suggested that large volumes of pre-Cenozoic dolomites reflect dolomitization at elevated burial temperatures of these rocks, whereas most Cenozoic carbonate platforms did not reach sufficient thermal maturity. A third, hybrid model suggests that Mg-rich dolomite precursor minerals precipitated in shallow environments and later underwent deep diagenesis to a more ordered and stoichiometric dolomite. The combination of carbonate oxygen (δ¹⁸O) and clumped (TΔ₄₇) isotope analysis can be used to constrain and distinguish among these formation environments.

Here, we combine δ¹⁸O and TΔ₄₇ measurements in marine carbonate rocks from the Triassic Ramon Gr. in Makhtesh Ramon, southern Israel, to constrain their formation environments. The studied section records a transition from a carbonate platform, dominated by fossil-rich limestone (top Gvanim and Saharonim Fm.), to a shallow saline evaporitic lagoon (Mohila Fm.) dominated by alterations of laminar dolomite and evaporitic gypsum, with much sparser fossils relative to top Gvanim and Saharonim Fm. Calcite samples in the Gvanim and Saharonim Fm. recorded δ¹⁸O and TΔ₄₇ values from -8.41 to -2.17 ‰ VPDB, and from 29 to 98 °C, respectively. Two calcite samples recorded TΔ₄₇ values of 152-231 °C, associated with isotopic solid-state reordering in response to local heating near igneous intrusions. Dolomite samples at the top Saharonim and Mohila Fms. recorded δ¹⁸O and TΔ₄₇ values from -4.77 to -1.59 ‰ VPDB and from 36 to 74 °C, respectively. These results indicate that carbonate minerals recrystallized in burial-diagenetic environments in an open system with respect to δ¹⁸O. The observation that dolomite, associated by stratigraphic context and texture with deposition at (or near) the surface, has been recrystallized at depth, supports a multi-step dolomite formation process, in which carbonates were first enriched in Mg²⁺ in the lagoon and later recrystallized in high-temperature, deep-diagenetic environment.